XL] PHILOSOPHY OF INDUCTIVE INFERENCE. 237 



be certain of something else ; for if this gold and nitric 

 acid remain what they were, I may be sure there will be 

 no solution on again trying the experiment. If I take other 

 portions of gold and nitric acid, and am sure that they really 

 are identical in properties with the former portions, I can 

 be certain that there will be no solution. But at this point 

 my knowledge becomes purely hypothetical ; for how can I 

 be sure without trial that the gold and acid are really 

 identical in nature with what I formerly called gold and 

 nitric acid. How do I know gold when I see it ? If I 

 judge by the apparent qualities colour, ductility, specific 

 gravity, &c., I may be misled, because there may always 

 exist a substance which to the colour, ductility, specific 

 gravity, and other specified qualities, joins others which we 

 do not expect. Similarly, if iron is magnetic, as shown by 

 an experiment with objects answering to those names, then 

 all iron is magnetic, meaning all pieces of matter identical 

 with my assumed piece. But in trying to identify iron, I 

 am always open to mistake. Nor is this liability to mis- 

 take a matter of speculation only. 1 



The history of chemistry shows that the most confident 

 inferences may have been falsified by the confusion of one 

 substance with another. Thus strontia was never discri- 

 minated from baryta until Klaproth and Haiiy detected 

 differences between some of their properties. Accordingly 

 chemists must often have inferred concerning strontia 

 what was only true of baryta, and vice versd. There is 

 now no doubt that the recently discovered substances, 

 caesium and rubidium, were long mistaken for potassium. 2 

 Other elements have often been confused together for 

 instance, tantalum and niobium ; sulphur and selenium ; 

 cerium, lanthanum, and didymium ; yttrium and erbium. 



Even the best known laws of physical science do 

 not exclude false inference. No law of nature has been 

 better established than that of universal gravitation, and 

 we believe with the utmost confidence that any body 

 capable of affecting the senses will attract other bodies, 

 and fall to the earth if not prevented. Euler remarks 



1 Professor Bowen has excellently stated this view. Treatise on 

 Logic. Cambridge, U.S.A., 1866, p. 354. 



2 Roscoe's Spectrum Analysis, ist edit., p. 98. 



